Wireless systems such as the ones based on the IEEE 802.11 standard for indoor local area communication, and the Third Generation Partnership Project (3GPP) based Long Term Evolution (LTE) for terrestrial cellular communication use multiple antennas for transmission and reception, both at an access point (e.g., base-station) and a user terminal (e.g., user equipment (UE)). These systems typically operate over a wider transmission bandwidth, use multiple-input and multiple-output (MIMO) technology, and employ broadband modulation formats such as OFDM and/or SC-FDMA. Since data processing can occur in the frequency-domain (unlike the time-domain data processing employed in Second Generation (2G), such as Global System for Mobile Communications (GSM), and Third Generation (3G), such as Wideband Code Division Multiple Access (WCDMA)), OFDM/SC-FDMA modulation schemes can convert a frequency-selective channel into multiple frequency-flat channels within the transmission bandwidth. The result is that, instead of complex channel estimation and equalization algorithms needed in time-domain, subcarrier-level frequency-flat nature of the OFDM/SC-FDMA modulations allows the receiver processing to be performed in frequency-domain.
Channel estimation may be performed by having a transmitter send known (or pilot) symbols to a receiver, which estimates the channel characteristics using the known symbols, and feeds back to the transmitter the estimated channel characteristics via a dedicated channel. The transmitter may then use the estimated channel characteristic, for example, in adaptive transmission techniques to improve bit rates. For example, the downlink reference signal received power (RSRP), reference signal received quality (RSRP) are the two well-known metrics the UE reports to eNodeB. Additionally, the UE transmits quantized downlink channel quality information (CQI) to eNodeB on periodic/aperiodic basis, either in the physical uplink shared channel (PUSCH) or physical uplink control channel (PUCCH). The CQI information in LTE systems can be sub-divided into wideband CQI, sub-band CQI, and UE-configurable sub-band CQI.
For UE with a single transmitting antenna, multiple antennas at the base-station (e.g., a single-input and multiple-output (SIMO) configuration) can be used to provide interference rejection capability. An example of spatial-division multiple access (SDMA) includes more than one UE transmits simultaneously to a base-station, and a base-station with multiple receiver antennas, which can jointly decode signals transmitted by the multiple UEs. This simultaneous transmission and joint detection technique can also be referred to as multi-user (MU) MIMO. In MU MIMO, each UE can have more than one transmitter antenna, and more than one transmission stream.